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and the Theory of Chemical Types. 
The law of substitutions sees then in these five compounds 
some of the nearest^ the most necessary modifications of ether. 
The theory of equivalents sees there any modifications what- 
ever more or less possible. The one says, these five bodies 
must be formed first very easily and very abundantly ; the 
other says, they may be formed, and many others with them. 
If the acetic acid is in question, the theory of equivalents 
would besides announce the possible formation of compounds 
so numerous, that nothing could guide the observer. The 
law of substitutions, more precise, foresees and predicts that 
in losing 1, 2, 3 equivalents of hydrogen, the acetic acid will 
take 1, 2, 3 equivalents of chlorine, and will thus produce 
three new compounds. One of them constitutes the so-called 
chloracetic acid. 
In a multitude of possible actions which are nearly equally 
foreseen by the theory of equivalents, the lawr of substitutions 
discovers with certainty those which are about to be produced ; 
it foresees them, predicts them, and up to tlie present time its 
help has truly been of invaluable efficacy. 
How, without it, should we have been led to discover, one 
after the other, four or five mixed products, hardly differing 
from each other, in some actions recently studied ? Other- 
wise, how would it have been possible to perceive, that the 
action which was to be produced had not been exhausted, if 
the formulae, through the impossibility of making them agree 
with the law of substitutions, had not warned the observer ? 
Let me make a comparison drawn from a familiar order of 
ideas. Let us put ourselves in the place of a man overlooking 
a game at chess, without having the slightest knowledge of 
the game. He would soon remark, that the pieces must be 
used according to positive rules. In chemistry, the equiva- 
lents are our pieces, and the law of substitutions one of the 
rules which presides over their moves. And as in the oblique 
move of the pawns, one pawn must be substituted for another, 
so, in the phenomena of substitution one element must take 
the place of another. But this does not hinder the pawn 
from advancing without taking anything, as the law of substi- 
tutions does not hinder an element from acting on a body 
without displacing or taking the place of any other element 
which it may contain. 
How can we believe that a knowledge of the rules which 
govern the game of our chess-board, is useless for the ex- 
planation of the moves which offer themselves, for the purpose 
of foreseeing those which are about to arise from the rela- 
tions of the different pieces, of the various agents placed in 
contact ? 
